Streptococcus pneumoniae are Gram-positive, lancet shaped cocci that are usually seen in pairs (diplococci), but also in short chains or as single cells. They grow readily on blood agar plates with glistening colonies and display alpha hemolysis unless grown anaerobically where they show beta hemolysis. The cells of most pneumococcal serotypes have a capsule which is a polysaccharide coating surrounding each cell. This capsule is a determinant of virulence in humans, as it interferes with phagocytosis by preventing antibodies from attaching to the bacterial cells. Currently there are more than 90 known pneumococcal capsular serotypes identified, with the 23 most common serotypes accounting for approximately 90% of invasive disease worldwide. As a vaccine, the pneumococcal polysaccharide coat can confer a reasonable degree of immunity to Streptococcus pneumoniae in individuals with developed or unimpaired immune systems, but the capsular polysaccharide conjugated to a suitable carrier protein allows for an immune response in infants and elderly who are also at most risk for pneumococcal infections.
Since the introduction of the first 7-valent pneumococcal conjugate vaccine (PCV7 or Prevnar) in 2000, invasive disease from those seven serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F) has nearly disappeared. The addition of serotypes 1, 3, 5, 6A, 7F and 19A in Prevnar 13 further decreased the numbers of invasive pneumococcal disease.
None of the currently marketed pneumococcal vaccine provides an appropriate protection against serotype 10A, 22F or 33F Streptococcus pneumoniae in human and in particular in children less than 2 years old. Therefore, there is a need for immunogenic compositions that can be used to induce an immune response against serotype 10A, 22F or 33F Streptococcus pneumonia. 